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    SubjectRe: [RFD] Layering: Use-Case Composers (was: DRBD - what is it, anyways? [compare with e.g. NBD + MD raid])
    Evgeniy Polyakov wrote:
    > Al Boldi ( wrote:
    > > Look at ZFS; it illegally violates layering by combining md/dm/lvm with
    > > the fs, but it does this based on a realistic understanding of the
    > > problems involved, which enables it to improve performance, flexibility,
    > > and functionality specific to its use case.
    > >
    > > This implies that there are two distinct forces at work here:
    > >
    > > 1. Layer components
    > > 2. Use-Case composers
    > >
    > > Layer components should technically not implement any use case (other
    > > than providing a plumbing framework), as that would incur unnecessary
    > > dependencies, which could reduce its generality and thus reusability.
    > >
    > > Use-Case composers can now leverage layer components from across the
    > > layering hierarchy, to yield a specific use case implementation.
    > >
    > > DRBD is such a Use-Case composer, as is mdm / dm / lvm and any fs in
    > > general, whereas aoe / nbd / loop and the VFS / FUSE are examples of
    > > layer components.
    > >
    > > It follows that Use-case composers, like DRBD, need common functionality
    > > that should be factored out into layer components, and then recompose to
    > > implement a specific use case.
    > Out of curiosity, did you try ndb+dm+raid1 compared to drbd and/or zfs
    > on top of distributed storage (which is a urprise to me, that holy zfs
    > suppors that)?

    Actually, I may not have been very clear in my Use-Case composer description
    to mean internal in-kernel Use-Case composer as opposed to external Userland
    Use-Case composer.

    So, nbd+dm+raid1 would be an external Userland Use-Case composition, which
    obviously could have some drastic performance issues.

    DRBD and ZFS are examples of internal in-kernel Use-Case composers, which
    obviously could show some drastic performance improvements.

    Although you could allow in-kernel Use-Case composers to be run on top of
    Userland Use-Case composers, that wouldn't be the preferred mode of
    operation. Instead, you would for example recompose ZFS to incorporate an
    in-kernel distributed storage layer component, like nbd.

    All this boils down to refactoring Use-Case composers to produce layer
    components with both in-kernel and userland interfaces. Once we have that,
    it becomes a matter of plug-and-play to produce something awesome like ZFS.



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